In many areas of material and construction technology a study of the properties of work pieces under stress is of great importance because such studies make statements about the properties of the serviceability and fatigue of the work piece possible. Such stress studies are not only performed on work pieces that consist of homogenous material, but also on work pieces of heterogenous materials such as fiber-reinforced plastics, etc.; in addition, the stress properties of connections (glued and screwed connections, weld joints) can be examined in multi-component work pieces.
It is already known to perform so-called stress-strain tests when investigating properties of work pieces under stress. In a stress-strain test the work piece is acted upon with increasing tensile stress and the resulting extension of the work piece measured. The extension can be measured, for example, by a wire strain gauge stuck onto the work piece or by a clip-on extensometer that has been clamped onto it--designated also as a "set extension sensor" Stress-strain tests can be carried out without damaging the object or until breakage occurs. They make possible a quasi-static characterization of the properties of the work piece under tension, but do not provide any information about the fatigue strength of the work piece.
To examine the fatigue strength of a work piece it is already known to perform a so-called hysteresis measurement by applying cyclical dynamic stress to the work piece in the tensile or compression area in accordance with the specifications of claim 1 and determining the mechanical-dynamic properties of the work piece and their change during the dynamic stress (see FIG. 1). Through such hysteresis measurements statements concerning damping, stiffness, and non-elastic deformation properties, in particular with polymer materials, can be made, as well as statements concerning the plastic extension properties of the work piece that cannot be obtained by clear stress-strain tests. In this regard it is of special practical significance that practical evaluative criteria concerning the fatigue properties of the work piece under real conditions of use can be collected from such mechanical-dynamic measurable characteristics of the work piece and that statements about the onset and the course of the damaging phenomena in the work piece, such as, for instance, micro-cracks, can be gathered.
As with clear stress-strain tests, hysteresis measurements can also be carried out until the work piece breaks.
In summary, it can be said in general that dynamically stressed piece parts can be considerably better characterized and more safely designed by using hysteresis measurements.
In the case of known hysteresis measurements a wire strain gauge or a clip-on extensometer is used to measure the strain.
Use of the wire strain gauge has proven disadvantageous in that it has only a relatively limited measurement area and thus is not suitable for all investigations.
Clip-on extensometers have a considerably larger measurement area (about 100% relative extension and better). Because the two measurement points between which a length change (extension) of the work piece is measures are made by cutting edges that are clamped onto the testing objects in the case of a clip-on extensometer, the danger that the cutting edge will slip arises especially with hysteresis measurements--a situation that can lead to a falsification of the measurement results with respect to the values of the interval. The effects of friction between the cutting edges and work piece can also lead to false statements, e.g., in the case of polymer materials with respect to mechanical damping. Moreover, it is unfavorable that damage to the surface of the work piece can occur through the mechanical contact between cutting edge and work piece and for this reason effects on the fatigue strength cannot be ruled out.
A fundamental disadvantage of determining the extension both by using wire strain gauges and clip-on extensometers consists in the fact that these sensors always make possible an integral measure of the extension between two fixed measurement points. In other words, the extension property is not determined outside of the measurement points, and the extension between the measurement points can only be determined with the set, specified definition given by the distance between the measurement points. Because in general only a clip-on extensometer is clamped onto the work piece due to space considerations, measurements at multiple sites are usually not possible. Different workpiece extension properties that are a function of the place therefore cannot be determined for this reason or are only capable of determination on a very limited basis.
From EP 0194354, the use of a laser measuring procedure with clear tensile tests on a sample to measure relative length changes of a sample without contact to the object is already known; in this laser measuring procedure a laser beam continuously scans the sample in the direction of draw and is reflected on a raster attached to the sample, and the reflected laser beam, whose intensity is modulated by the raster, is registered by a photodetector, through which the change in the sample's extension can be inferred by measuring the modulation frequency of the reflected laser beam.
The task of the invention is to develop the hysteresis procedure described in the introduction in a way that considerably extends the possibilities of characterizing a work piece subjected to dynamic stress known up to now, which makes possible statements about the workpiece fatigue strength and the fatigue properties, as well as possible damage processes--which can be of considerable importance to designing and dimensioning the work piece and which could not be obtained in a comparable form until now. It is, moreover, a further task of the invention to create a device through which the continuing characterization possibilities of the work piece mentioned are realized and can be distributed in suitable form.
The claimed subject matter is intended to solve these tasks.
Different sections of the work piece can be examined at the same time with respect to their mechanical-dynamic properties through the procedure according to the invention, as a result of which, for example, the determination of a local material nonhomogeneity of the work piece due to anomalous damping, stiffness, or deformation properties are made possible in one of the observed sections. In this way a weak spot that can occur in the work piece with dynamic stress can be localized and characterized in connection with different dynamic stresses. In addition, the existence of nonhomogeneities and their local spreading can be followed by monitoring the local dynamic stress properties of the work piece over time with the assistance of characteristic values measured for the different sections. In this way the fatigue properties of the work piece with respect to time and place can be characterized specific to stress. For example, a multi-component work piece that is connected by a weld seam can be examined locally in the seam area for solidification or loss of cohesion (especially with metals) under external dynamic stress. Such statements are of great interest when one is interested in testing different work pieces to determine their capability for a given use or when one is faced with the task of improving a given work piece in a way that does justice to the given demands with respect to its stability properties.
A further advantage of the procedure according to the invention consists in the fact that the workpiece sections that are to be investigated are variable--i.e., before each measurement both their position and length can be freely defined. As a result, the simple adjustment of the procedure according to the invention to different measuring demands (e.g., the demanded spatial definition of the measurement) and a reduction of the obtained test data are made possible.
Due to its property as a local investigative procedure, the procedure according to the invention is especially well suited for the investigation of multi-component work pieces or heterogenous work pieces with locally differing structures.
Instead of laser beams, a collimated light beam from a non-laser scanning source can be used for scanning the raster.
When the direction of the cyclical tensile and/or compression stress (first direction) coincides with the direction of the raster and the laser scanning (second direction), the measurement of the characteristic value(s) occurs directly in the direction in which the power is introduced. The direction of the longitudinal extension of the raster--i.e., the direction vertical to the coding strips that make up the raster--is designated by the direction of the raster.
Because the laser beam has a finite scanning speed, the values for the length changes of the monitored sections are recorded at different points in time by a laser scan. To rule out this undesired effect and increase measuring precision it is preferred to assign a particular absolute reference time (t.sub.i) to the distance signal of each laser scan within the framework of the evaluation and to calculate the length changes of the different sections with respect to the assigned fixed reference time (t.sub.i) for each laser scan.
In accordance with an appropriate variation of the model of the present invention, for each workpiece section the distance signal (which, if applicable, is previously formed by impulses) is transformed into a digital signal of the section length change that represents the length change of the corresponding section between two successive laser scans. These digital signals of the section length changes assigned to the respective sections can then be fed to a sequential computer, which determines one or several of the characteristic values of each workpiece section by means of a stored computer program while taking into consideration the digitized power signal from an A/D converter.
In this case the correction of the signal of the section length change obtained during a single laser scan that was already mentioned can be carried out by a calculation of the computer program. It calculates the corresponding corrected values valid for the fixed reference time t.sub.i from the values of the signals of the section length change obtained during the scan by means of an interpolation procedure. The characteristic values are then determined on the basis of these corrected values.
A comparison of the characteristic values determined for different sections of the work piece--for example through difference formation--makes possible considering the changes of the characteristic variables along the path of the raster on the work piece.
Moreover, the properties of the curves of the damping distribution and/or the tensile and compression stiffness and/or the non-elastic deformation and/or the plastic extension over time can be followed by using a graphics program in real time stored in the sequential computer; and those curves can be represented graphically, for example, by transforming the work piece into a false color representation.
Although in principle cyclical stress can be carried out alone in the tensile or compression area, the preferred method is to exert cyclical stress by alternating between the tensile and compression areas because in this way different properties in the tensile and compression areas can be determined and characterized in a single experiment.
The dynamic path of an oscillation cycle can be varied in a wide range, and it has been proven that especially sine-shaped and triangular or rectangular dynamic paths create favorable conditions for the mechanical-dynamic investigation of a work piece.
According to an especially preferred working model of the procedure, the work piece is exposed to a time sequence of different stress levels, which are superimposed by cyclical tensile and/or compression stress (i.e., tensile or compressional vibration). Through such so-called stress-increase tests information can be obtained about the stress-dependent properties (for example, with respect to the transition from the linear visco-elastic area at the low stress levels that occur with polymers to the nonlinear visco-elastic area at higher stress levels; and statements can be made about the onset of irreversible damage to the work piece. In addition, by the cyclical repetition of the stress-increase test, clues can be obtained about the speed of deterioration and by comparing the characteristic values obtained at each repetition of the sequence of the stress level with respect to the same stress level each time, it is possible, in the meantime, to follow the effect of low or high stress on irreversible damage.
According to an especially preferred variation of the stress-increase test, the stress can be brought back down to a basic stress level with a low stress value after each stress level and the comparison of the characteristic value(s) mentioned can be carried out at this basic stress level. In this way possible damage caused in the meantime by higher stress can be exactly recognized and analyzed.
According to another advantageous variation of the model of the invention the procedure can be used, moreover, to determine the transversal contraction of the work piece by having a stationary pencil of light, emitted from a light source, partially shaded by a cross dimension that is larger than the cross dimension of the work piece, whereby the intensity of the remaining light that is not shaded by the work piece is captured by a detector and transformed into a signal characterizing the cross dimension of the work piece. This, for example, makes possible determining the transversal contraction of the work piece by varying the tensile stress.
If the extension of the light spot produced from the pencil of light on the work piece in the second direction (i.e., the direction of the raster) essentially corresponds to the length of one or several of the workpiece sections under consideration, determining the transversal contraction of the work piece in the area of one or several of the sections under consideration becomes possible.
A refinement of the cross dimension measurement of the work piece can be obtained when the work piece is equipped with another raster made of contrasting coding strips in a third direction and another laser is used whose laser beam sweeps the additional raster; thus the extension properties of the work piece are determined locally in the third direction in a way similar to the way the extension in the first direction is measured. In this way the contraction or extension properties of the work piece can be obtained at the same time; and they can be obtained locally in the second and third direction (i.e., for instance, in the longitudinal and transverse direction to the direction from which the power is introduced) and put into relationship with each other--which can then be of interest if the cracks that form in fiber-reinforced material with longitudinally running fibers are to be analyzed.
Both the integral (if applicable, limited to a section) measurement of the cross dimension and the local measurement of the distribution of the cross dimension of the work piece by using the additional raster make determining Poisson's ratio--the ratio of the longitudinal extension to the transversal contraction--possible in a single experiment. In the case in which the distribution of the cross dimension is measured, the additional raster on the work piece can be attached to a side across from the first raster.
Through a possible local and/or temporary warming or cooling of the work piece the procedure according to the invention can be extended advantageously to the analysis of temperature-dependent phenomena. For the analysis of the mechanical-dynamic properties of the work piece under consideration warming without contact to the object is appropriate.
The problem that the present invention seeks to clear up is solved by the device according to claim 13.
Accordingly, the maximum spatial definition is determined by using the device to measure through the center distance of two successive coding strips, whereby one must, however, take into consideration the fact that with a reduction of the center distance, the thickness of the coding strip must necessarily be reduced, as a result of which the modulation intensity of the modulated laser beam decreases. This has the consequence that in practice a lower limit is specified for the center distance of two successive coding strips. It has been shown that favorable measuring conditions exist when the center distance lies in the range between 0.5 and 10 mm, and especially between 2 and 5 mm.
Within the framework of the stress-increase tests already mentioned, the data-processing device preferably has a computer program that compares the characteristic values obtained for the same stress levels with each repetition of the sequence of stress levels.
Further advantageous refinements of the procedure according to the invention and the device according to the invention can be inferred from the subclaims. The invention is described by using examples with the help of the drawings below. The following figures are used.